This invention relates to silicone compositions capable of adhering to plastic film substrates independent of the type of plastic film, and release films using the same.
In the coating art, there are known a variety of silicone release compositions for use in preventing any sticking or adhesion between substrates of paper, plastics or the like and pressure-sensitive adhesives. Of these, solvent type silicone compositions have been widely used because of release properties and relatively low substrate selectivity.
However, the demand for solventless silicone compositions is now increasing from the safety and health standpoints. A number of solventless silicone compositions are known in the art as described in JP-A 49-47426, JP-A 50-141591, JP-B 52-39791 and JP-A 57-77395. These compositions adhere well to paper and laminated paper, but do not adhere well to plastic films.
It was then proposed to blend certain silanes or siloxanes in organopolysiloxane for imparting self-adhesiveness thereto (see JP-B 46-2187 and JP-B 52-9469). Since these compositions cure through condensation reaction, which requires elevated temperatures, they are unsuited for plastic films.
JP-A 63-27560 and JP-A 6-293881 disclose silicone compositions based on an organovinylpolysiloxane having branching points within the molecule, which adhere well to polyolefin film substrates without a need for solvents. However, these silicone compositions raise the problem that they fail to establish satisfactory bonds when applied to polyester film substrates.
An object of the invention is to provide a silicone composition which cures in the absence of a solvent into a coating that adheres well to plastic film substrates independent of the type of plastic film. Another object is to provide a release film using the same.
It has been found that when first and second organopolysiloxanes specified below are combined, the resulting silicone composition, despite a solventless system, is able to cure into a coating that firmly adheres to plastic film substrates independent of the type of plastic film.
The present invention provides a silicone composition comprising
(A) 100 parts by weight of a first organopolysiloxane having the following average compositional formula (1): 
xe2x80x83wherein R1 is alkenyl, R2 is a monovalent hydrocarbon radical, R3 is hydrogen or alkyl, m, n and p are such positive numbers in the range: 0.6xe2x89xa6(n+p)/mxe2x89xa61.5 and 0xe2x89xa6p/(n+p)xe2x89xa60.05 that the organopolysiloxane has a viscosity at 25xc2x0 C. of 5 to 100 mPaxc2x7s,
(B) 10 to 100 parts by weight of a second organopolysiloxane having the following general formula (2): 
xe2x80x83wherein R4 is a monovalent hydrocarbon radical, at least two of the entire R4 radicals are alkenyl, t is a number from 0 to 15, q, r, s and t are such numbers that the organopolysiloxane has a viscosity at 25xc2x0 C. of from 100 mPaxc2x7s to a 30% toluene dilute-solution viscosity of 20,000 mPaxc2x7s,
(C) 30 to 100 parts by weight of an organohydrogenpolysiloxane having at least two hydrogen atoms each directly attached to a silicon atom per molecule, and
(D) a catalytic amount of a platinum group metal catalyst.
The present invention also provides a release film in which a cured coating of the silicone composition is formed on a plastic film substrate.
(A) First Organopolysiloxane
The first organopolysiloxane is the most important component in the silicone composition of the invention and has the following average compositional formula (1). 
Herein R1 is alkenyl, R2 is a monovalent hydrocarbon radical, and R3 is hydrogen or alkyl. Suitable alkenyl radicals represented by R1 are those having 2 to 8 carbon atoms, such as vinyl, allyl, butenyl, pentenyl and hexenyl. Suitable monovalent hydrocarbon radicals represented by R2 are those having 1 to 8 carbon atoms, for example, alkenyl radicals such as vinyl, allyl, butenyl and pentenyl as exemplified for R1, alkyl radicals such as methyl, ethyl and propyl, aryl radicals such as phenyl, aralkyl radicals such as benzyl, and substituted ones of the foregoing radicals in which some or all of the hydrogen atoms are substituted with halogen atoms. Suitable alkyl radicals represented by R3 are those having 1 to 6 carbon atoms, such as methyl, ethyl and propyl.
The subscripts m, n and p are such positive numbers that the first organopolysiloxane has a viscosity at 25xc2x0 C. of 5 to 100 mPaxc2x7s, preferably 10 to 50 mPaxc2x7s. The value of (n+p)/m is from 0.6 to 1.5, and preferably from 0.8 to 1.2. If (n+p)/m less than 0.6, the adhesion to plastic film declines. If (n+p)/m greater than 1.5, which indicates a higher proportion of R2SiO3/2 units, it is difficult to synthesize such organopolysiloxanes. The value of p/(n+p) is from 0 to 0.05, and preferably from 0 to 0.03. If p/(n+p) greater than 0.05, which indicates a higher content of hydroxyl or alkoxy radicals, the resulting silicone composition becomes less curable.
As described above, the first organopolysiloxane has a viscosity at 25xc2x0 C. of 5 to 100 mPaxc2x7s. A viscosity of less than 5 mPaxc2x7s detracts from curability whereas a viscosity of more than 100 mPaxc2x7s detracts from adhesion.
The first organopolysiloxane should preferably have a vinyl value of 0.5 to 2.25 mol/100 g, and more preferably 0.5 to 1.0 mol/100 g.
The first organopolysiloxane can be prepared, for example, by subjecting a trialkoxymethylsilane and a dialkenyltetramethyldisiloxane or hexamethyldisiloxane in an alcohol solvent to cohydrolysis in the presence of an acid catalyst, neutralizing, removing the alcohol by-product, washing with water, and removing the unreacted reactants.
(B) Second Organopolysiloxane
The second organopolysiloxane is a component that largely governs the release force of a cured coating of the silicone composition. By altering the structure and substituents of this component, the release properties of the cured coating can be controlled. The second organopolysiloxane has the following general formula (2). 
Herein R4 is a monovalent hydrocarbon radical, preferably those radicals of 1 to 8 carbon atoms, as exemplified above for R2. At least two of the entire R4 radicals should be alkenyl.
The subscripts q, r, s and t are such numbers that the second organopolysiloxane has a viscosity at 25xc2x0 C. in a range from 100 mPaxc2x7s to a 30% toluene dilute-solution viscosity of 20,000 mPaxc2x7s, and especially in a range from 200 mPaxc2x7s to a 30% toluene dilute-solution viscosity of 5,000 mPaxc2x7s, while t is in the range: 0xe2x89xa6txe2x89xa615, and especially 0xe2x89xa6txe2x89xa66. If the viscosity of the second organopolysiloxane is less than 100 mPaxc2x7s, the resulting silicone composition becomes less viscous and less effective to apply to film substrates. If the 30% toluene dilute-solution viscosity exceeds 20,000 mPaxc2x7s, the resulting silicone composition becomes too viscous and less efficient to apply.
The second organopolysiloxane may be used singly or in admixture of two or more. Preferably, the second organopolysiloxane contains 10 to 50% by weight based on the entire second organopolysiloxane of an organopolysiloxane having a 30% toluene dilute-solution viscosity of 1,000 to 10,000 mPaxc2x7s at 25xc2x0 C. and especially 2,000 to 5,000 mPaxc2x7s at 25xc2x0 C., and a vinyl value in average molecule of at least 0.02 mol/100 g, more preferably 0.02 to 0.2 mol/100 g, and even more preferably 0.02 to 0.08 mol/100 g. The preferred second organopolysiloxane ensures that the resulting silicone composition provides a cured coating having satisfactory adhesion, release properties and wear resistance. It is preferred to use an organopolysiloxane having a viscosity at 25xc2x0 C. of 200 to 1,000 mPaxc2x7s and a vinyl value of 0.01 to 0.10 mol/100 g as the remainder of the second organopolysiloxane.
An appropriate amount of the second organopolysiloxane blended is 10 to 100 parts by weight, and preferably 10 to 50 parts by weight, per 100 parts by weight of the first organopolysiloxane. With less than 10 parts of the second organopolysiloxane, the release force of cured coatings increases beyond the practical level. More than 100 parts of the second organopolysiloxane detracts from the adhesion of cured coatings to plastic film substrates.
(C) Organohydrogenpolysiloxane
The organohydrogenpolysiloxane used herein has at least two hydrogen atoms each directly attached to a silicon atom (i.e., SiH radicals) per molecule. Addition reaction takes place between SiH radicals in the organohydrogenpolysiloxane and alkenyl radicals in the first and second organopolysiloxanes whereby the composition cures, that is, a cured coating forms. Preferred organohydrogenpolysiloxane has the following average formula (3).
R5aHbSiO(4xe2x88x92axe2x88x92b)/2xe2x80x83xe2x80x83(3) 
Herein R5 stands for substituted or unsubstituted monovalent hydrocarbon radicals, preferably such radicals of 1 to 8 carbon atoms and free of aliphatic unsaturation, for example, alkyl radicals such as methyl, ethyl and propyl, aryl radicals such as phenyl, and halo-alkyl radicals such as 3,3,3-trifluoropropyl. The subscripts a and b are numbers in the range: 0.5xe2x89xa6axe2x89xa61.5, 0.5xe2x89xa6bxe2x89xa61.5 and 1.0xe2x89xa6a+bxe2x89xa63.0, and preferably 0.8xe2x89xa6axe2x89xa61.0, 0.8xe2x89xa6bxe2x89xa61.0 and 1.6xe2x89xa6a+bxe2x89xa62.0.
The amount of the organohydrogenpolysiloxane blended is determined in accordance with the amount of alkenyl radicals included in the first ad second organopolysiloxanes. From the standpoints of cure and release properties of coatings, an appropriate amount is usually 30 to 100 parts by weight per 100 parts by weight of the first organopolysiloxane. Less than 30 parts of the organohydrogenpolysiloxane leads to under-cure whereas more than 100 parts increases the release force beyond the necessity.
(D) Platinum Group Metal Catalyst
The platinum group metal catalyst serves to promote the addition reaction of the first and second organopolysiloxanes with the organohydrogenpolysiloxane. Any of well-known addition reaction catalysts is useful. Suitable platinum group metal catalysts include platinum, palladium and rhodium base catalysts, with the platinum base catalysts being preferred. Examples of the platinum base catalysts include chloroplatinic acid, alcohol and aldehyde solutions of chloroplatinic acid, and complexes of chloroplatinic acid with olefins and vinylsiloxane.
The platinum group metal catalyst is added in a catalytic amount. For obtaining satisfactory cured coatings and from the economical standpoint, it is preferred that the platinum group metal catalyst be added in an amount of 1 to 1,000 ppm of platinum group metal based on the total amount of the first and second organopolysiloxane and the hydrogenpolysiloxane.
The silicone composition of the invention is obtained by formulating predetermined amounts of the above-described components (A) to (D). Optional components may be added to the essential components. Suitable components which can be added if desired include activity suppressing agents such as organic nitrogen compounds, organic phosphorus compounds, acetylene compounds, oxime compounds and organic chlorine compounds useful for the purpose of suppressing the catalytic activity of the platinum group metal catalyst; and silicone resins, silica and organopolysiloxanes free of silicon atom-attached hydrogen atoms (SiH radicals) and alkenyl radicals useful for the purpose of controlling the release force. The optional components may be added in conventional amounts as long as this does not compromise the objects of the invention.
In the preferred process of preparing the silicone composition according to the invention, components (A) to (C) and optional components, if necessary, are intimately premixed before component (D) is added thereto. It is noted that for each component, one or more compounds may be used. If desired, the silicone composition can be diluted with an organic solvent such as toluene or hexane prior to use.
The silicone composition thus prepared is applied to a plastic film substrate and heat cured thereto, giving a release film. Suitable examples of the plastic film substrate include polyolefin films such as biaxially oriented polypropylene films, polyethylene films and ethylene-propylene copolymer films, and polyester films. The thickness of such film substrate is not critical although film substrates of about 5 xcexcm to about 100 xcexcm thick are preferably used. The coating weight of the silicone composition on the film substrate is not critical although a coating weight of about 0.1 to about 2.0 g/m2 is usually employed.
Preferably the silicone composition is cured by heating at a temperature of about 50xc2x0 C. to about 200xc2x0 C. for a time of about 1 second to about 5 minutes.